1. Field of the Invention the Invention
This invention relates to methods and apparatus for measuring and displaying the phase relationship between the subcarrier and the horizontal sync (the SC/H phase) of a video signal, and in particular to a high accuracy digital method and apparatus for measuring and displaying the SC/H phase of a plurality of video signals simultaneously.
2. Prior Art
Color video display apparatus have discrete phosphor areas disposed in a repetitive pattern of colors, the individual dots or segments for a particular color being activated by the electron beam as it scans over the screen in horizontal lines. The video color information encoded in the video signal is presented in timed coincidence with the scanning of the beam over the repetitive color patterns, and accordingly the phase relationship between the encoded color information and the sync pulses which trigger the scanning circuits of the display apparatus is an important consideration. Color information is synchronized to a subcarrier frequency defined by the color burst pulse which occurs shortly after the horizontal sync pulse for each horizontal line. Test equipment is available for measuring and displaying the phase relationship between the subcarrier and the horizontal sync pulse, commonly known as the SC/H phase, for use by video production personnel, broadcasters, etc.
Known devices for SC/H phase measurement and display typically rely on variable analog devices for providing phase delays, ramp signals and the like. As a result, the only practical way to measure the SC/H phase of a plurality of video signals simultaneously has been to substantially duplicate the measurement and display apparatus for every signal which is to be measured. Multiple circuits, however, do not act identically due to variations in components and the like. The multiple circuits can be expected to provide at least slightly different results even for one signal. Moreover, the analog devices are subject to problems of accuracy and drift.
According to one known method of SC/H phase measurement, one subcarrier signal is phase-locked to the horizontal sync and another subcarrier signal is phase-locked to the color burst. The SC/H phase difference is defined by the phase difference of these two subcarriers, and is displayed, for example, by using the signals as the X and Y inputs to an oscilloscope to provide a vectorscope type phase display. The phase difference is represented by the angular displacement of the displayed dot from a reference dot or line representing a zero phase angle. Prior art devices along these lines are disclosed in U. S. Pat. Nos. 4,587,551--Penney; 4,788,585--Suzuki; and 4,694,324--Matney. Of course it is also possible to display the extent of phase difference between two signals as a numeric readout or as a bargraph.
According to another known method, the time lapse between the midpoint of the leading edge of the horizontal sync pulse and a zero crossing of the color burst is measured. Alternatively the time lapse can be measured between the leading edge of the sync and the next zero crossing of a reference subcarrier regenerated from the color burst. This normally requires an analog circuit or a combination analog/digital circuit, for finding the midpoint of the leading edge and determining the time lapse. The time lapse can be translated into degrees of phase difference by relating the time to the period of the subcarrier, e.g., at the nominal 3.579545 MHz for an NTSC subcarrier. The results can be displayed as a numeric readout, bargraph, etc.
U. S. Pat. No. 4,758,890--Boyce discloses a circuit for comparing the subcarrier frequency to a horizontal sync pulse. The sync pulse is delayed by a variable delay circuit controlled in a feedback loop, and operable to delay the sync pulse to ensure that the leading edges of the sync pulse corresponds to a the zero crossing of the subcarrier.
U. S. Pat. Nos. 4,603,346--Melling, Jr.; 4,680,620--Baker et al; and 4,792,845--Judge are additional examples of phase comparators and displays. U. S. Pat. No. 4,470,064--Michener discloses a circuit operable to capture and digitize the value of a burst-locked subcarrier and a quadrature subcarrier as a means of acquiring data necessary to calculate the SC/H phase. Additionally, the Tektronix model VM-700 Video Measurement Set calculates SC/H phase from a digitized signal. The VM-700 and the Michener device employ digital techniques, and Michener is switchable between two video inputs. Nevertheless, the devices can only deal with one signal at a time.
If a transmitted signal at one SC/H phase is suddenly substituted with a signal at a different SC/H phase, the result is a flash of poor color purity and/or a variation in intensity. It would be desirable to provide an accurate and drift free measurement apparatus for SC/H phase measurement that can measure and display multiple video sources simultaneously, i.e., without the need for the operator to switch between video sources for one-at-a-time measurements. Such a system would be particularly helpful as a production and broadcast tool in conjunction with video switchers coupled to multiple video sources such as a plurality of prerecorded sources and the like, assisting in the merging of multiple signal sources into a program.
The present invention provides a digital circuit operable to accomplish multiple source SC/H phase measurements, and provides a means for simultaneous display of the results in a preferably graphic format. In this manner the phase conditions of a plurality of sources can be monitored.